Hydraulic and pneumatic drilling machines comprise a striker member, e.g. a percussion piston, to transfer shock waves to an impact receiving member, e.g. a shank, which transfers these to the drill rod that via the boar crown penetrates the rock.
A percussion piston preferably strikes using a frequency of approximately 40-100 Hz and the stroke rate for the percussion piston is approximately 10 m/s, which thereby is subjected to high stresses.
If, for example, the percussion piston is exchanged after approximately 1000 hours it is during that time subjected to many load changes, which increases the risk for fatigue failure. It would be advantageous to increase the stroke rate to 12.5-13 m/s.
There are numerous ways to design the impact surface of the percussion piston. A number of known designs are schematically illustrated in FIGS. 2a-2c. 
In FIG. 2a is shown a percussion piston having a plane impact surface and provided with a radius transition of 2 (shown in the figure) or 3 mm (R2, R3) to the side surface.
As an alternative, a chamfer angled in relation to the impact surface is provided, where the angle is within the interval of 15-45 degrees. This is illustrated in FIG. 2b. 
According to still another alternative percussion pistons are provided with a radius covering the entire surface having a radius transition in the interval of 200-1000 mm (R200-R1000). This alternative is illustrated in FIG. 2c. 
The British patent document GB-324265 is disclosed a hammer rock drill comprising a percussion piston having an impact surface shaped such that the load on the moving part decreases due to a working tool being mounted out of alignment. Therefore, the impact surface of the percussion piston has a spherical concave shape and the shank has a corresponding spherical convex shape.
In the published patent application GB-2136725 a drill hammer provided with a striker is known, where the striker has a truncated cone shaped striker head, i.e. the transition between the side surface and the impact surface is chamfered.
In U.S. Pat. No. 6,273,199 an arrangement is disclosed applicable for rock drilling which includes a percussion piston and a shank.
And finally, the U.S. patent application US-2009/0133893 discloses a hand-held tool having a reciprocating percussion piston. The piston is provided with a spherical impact surface.
There exist both solid percussion pistons as well as percussion pistons provided with a central longitudinal opening.
The shank, to which the percussion piston transfers the shock wave, may be provided with a so called dowel hole at the surface hit by the percussion piston. The dowel hole is a centrally positioned hole which is related to the manufacture of the shank. The dowel hole may have a diameter of e.g. 8 mm.
The dowel hole incurs specific stresses upon the central parts of the impact surface of the percussion piston. Due to the large forces that the impact surface is subjected to it has been established that the central parts are subjected to material movements that briefly may be explained as the parts of the percussion piston above the dowel hole “moves” in the striking direction.
Herein it is important to mention that the shank wears out and replaces more often than the percussion piston.
In addition it has been established that due to wear out of e.g. bushings and so called guide sleeves the percussion piston does not hit the shank entirely straight in every strike. This results in high contact stresses at the contact surfaces.
Thus, in view of the above discussion of the prior art the object of the present invention is to achieve an improved design of the front part of the striker member that minimizes the stress concentration and thereby increases the life for the striker member which is economically favorable.